1. Field of the Invention
The present invention relates to a method of forming an alignment mark used in a lithography process, a substrate in which devices are formed, and a liquid discharging head using the substrate.
2. Related Background Art
Photolithography technology used for the formation of a desired pattern in a semiconductor device manufacturing process, and the like, is one in which a resist is applied on the substrate to perform baking, the resist on the substrate is masked and exposed with a photomask in which the desired pattern formed on the substrate is formed, development of the resist is performed, and then a material to be etched is removed by etching to form the desired pattern on the substrate. In the semiconductor device manufacturing process, the exposure, the development, and the etching are repeatedly performed with various photomasks because a complicated pattern is formed on the substrate. Therefore, in order to perform alignment between the substrate, in which a certain pattern is already formed, and the photomask, which is of a subsequent pattern original plate, it is necessary that alignment marks necessary to an alignment process using image processing are arranged at positions where the alignment marks of the substrate and the photomask are opposite to each other.
As shown in FIG. 6, in the semiconductor device manufacturing process, it is a mainstream technique to form a plurality of desired product devices 1008 from a substrate 1001, referred to as a wafer, in a collective manner. In a case wherein the plurality of product devices 1008 is formed from the substrate 1001 in a collective manner, the alignment mark for the alignment is arranged in an area 1007 dedicated to the alignment mark, which is different from the portion which becomes the product devices 1008.
FIG. 7 is an enlarged view showing the area 1007 dedicated to the alignment mark in an area A of FIG. 6 when viewed from the lower surface side of the substrate 1001 shown in FIG. 6. As shown in FIG. 7, a “cross-shaped” substrate-side alignment mark 1002 is formed in the area 1007 dedicated to the alignment mark located in the upper surface of the substrate 1001. A resist pattern 1005 is formed in the lower surface of the substrate 1001. The resist pattern 1005 includes alignment mark traces 1005a, which are arranged at the positions corresponding to four corners of the cross-shaped substrate-side alignment mark 1002. As shown in FIG. 7, the alignment mark traces 1005a are formed at the upper-right, lower-right, upper-left, and lower-left positions of the alignment mark 1002 so as not to overlap the alignment mark 1002. FIG. 7 is a perspective view showing the substrate-side alignment mark 1002 of the upper surface of the substrate 1001.
The technique of arranging the alignment mark for the alignment in the area dedicated to the alignment mark, which is different from the portion which becomes the product devices, is well known, without citing technical references.
However, when the alignment mark 1002 for the alignment is arranged in the area 1007 dedicated to the alignment mark, which is different from the area which becomes the product devices 1008, a yield of the product device 1008 obtained from the substrate 1001 is decreased by a size of the area 1007. Because the individual product device 1008 is obtained by dicing the substrate 1001 in which the plurality of product devices 1008 is formed, even if the size of the alignment mark 1002 is smaller than the area of the individual product device 1008, the area of each alignment mark 1002, which is not used for the product device 1008, becomes equal to the area of each product device 1008. Therefore, the decrease in yield becomes remarkable as the area of the product device 1008 is increased.
On the other hand, in order to eliminate the area which is not used for the product device, there is a well-known technique in which the alignment mark for the alignment is arranged in the area dedicated to the alignment mark, which is different from the area which becomes the product devices, but the alignment mark is provided within the area of the product device itself.
However, in the technique, the area of the individual product device is increased by the area necessary to arrange the alignment mark, so that the yield of the product device cannot be increased much.
Therefore, recently, the technique of arranging the alignment mark in the area where the substrate is cut (usually referred to as a “scribe line”) is used as one of the techniques of solving the problem that the yield of the product device is decreased due to the provision of the alignment mark. In accordance with the technique, since the alignment mark is given to the point which is cut in the dicing process, it is not necessary to separately reserve the area for the arrangement of the alignment mark on the substrate, which allows the substrate to be effectively utilized to increase the yield of the product device. For example, such a technique is described in Japanese Patent Application Laid-Open No. 2003-092246.
Referring to FIG. 8, this technique will be described. When a cutter (dicing blade) used for the cutting has a thickness of about 50 μm, the alignment mark 1002 can be arranged on a scribe line 1006 because of a fine rule of the current photolithography technology. However, in this case, since it is necessary to form more finely the alignment mark, it is necessary to use an apparatus which can perform the fine pattern forming with high accuracy, and such an apparatus is expensive. When the accuracy required for forming the alignment mark is higher than the accuracy required for forming the product device, manufacturing the product device with such accuracy leads to overspecifying the performance of the product device.
The above-described problem concerning the arrangement of the alignment mark remarkably appears when a through-hole piercing the substrate is particularly made as a necessary function of the product device. For example, in a process of manufacturing an ink jet recording head, ink discharging energy generation elements and nozzles are formed on a silicon (Si) substrate, and an ink supply port for supplying ink from the outside to each nozzle is caused to pierce the Si substrate. The through-hole is usually made by photolithography technology. In particular, Si anisotropic etching or dry etching with a reaction gas is used for making the through-hole. In either case, it is necessary to make the through-hole by performing the alignment between the ink supply port and a structure of the ink discharging energy generation element or nozzle, which is arranged in the substrate. However, the alignment mark formed on the Si substrate is also etched when the Si substrate is etched to make the through-hole. Therefore, sometimes, concave damage is generated in the portion where the alignment mark is formed in the Si substrate.
When the problem is solved by forming the alignment mark in the area which does not influence the area which becomes the product device, i.e., in the area dedicated to the alignment mark independent for the product device, as described above, the yield of the product device from the substrate is decreased by the area dedicated to the alignment mark.
When the technique of arranging the alignment mark on the scribe line 1006 is adopted, even if the alignment mark can be arranged on the scribe line 1006 in terms of size, since side etching (the etching spreads in a crosswise direction compared with the actual resist pattern) occurs from the portion where the alignment mark is formed during the etching, a defect is generated in the product devices arranged near four corners of the alignment mark. This is not preferable in regards to the quality of the product device.
Referring to FIG. 9, this phenomenon will be described. Assuming that the amount of side etching generated at the formation point of the alignment mark trace 1005a in the lower surface of the substrate 1001 is 50 μm in one side direction when the Si substrate 1001 is etched, the alignment mark trace 1005a has an original size of 50 μm by 50 μm (see FIG. 7) and a tilt angle of a <111> surface of the Si substrate 1001 by the Si anisotropic etching is 54.7°, so that, after the side etching, the portion where the alignment mark trace 1005a is formed becomes a groove 1009 having a shape of a quadrangular pyramid, in which an opening portion is 150 μm by 150 μm and a depth t is about 106 μm.
Broadening a width of the scribe line 1006 or forming more finely, the alignment is thought of as a technique of decreasing the influence of the side etching. However, the yield of the product device is decreased by the width of the scribe line 1006 when the width of the scribe line 1006 is broadened. When the alignment mark is formed more finely, there is a problem that the accuracy of the alignment is difficult to maintain in the fine alignment mark, while the cost of the alignment mark forming apparatus is increased. Particularly, in the semiconductor device manufacturing process, how much the yield of the product device is obtained from the wafer substrate largely influences the cost of the product device. Therefore, when the alignment mark used for the alignment between the substrate and the photomask is formed on the substrate, it is necessary to arrange the alignment mark so that the alignment mark decreases the yield of the product device.